ZIP3, a new splice variant of the PKC-ζ-interacting protein family, binds to GABAc receptors, PKC-ζ, and Kvβ2

The correct targeting of modifying enzymes to ion channels and neurotransmitter receptors represents an important biological mechanism to control neuronal excitability. The recent cloning of protein kinase C-zeta interacting proteins (ZIP1, ZIP2) identified new scaffolds linking the atypical protein kinase PKC-zeta to target proteins. GABA(C) receptors are composed of three p subunits (rho1-3) that are highly expressed in the retina, where they are clustered at synaptic terminals of bipolar cells. A yeast two-hybrid screen for the GABAc receptor rho3 subunit identified ZIP3, a new C-terminal splice variant of the ZIP protein family. ZIP3 was ubiquitously expressed in non-neuronal and neuronal tissues, including the retina. The rho3-binding region of ZIP3 contained a ZZ-zinc finger domain, which interacted with 10 amino acids conserved in rho1-3 but not in GABA(A) receptors. Consistently, only rho1-3 subunits bound to ZIP3. ZIP3 formed dimers with ZIP1-3 and interacted with PKC-zeta and the shaker-type potassium channel subunit Kvbeta2. Different domains of ZIP3 interacted with PKC-zeta and the rho3 subunit, and simultaneous assembly of ZIP3, PKC-zeta and rho3 was demonstrated in vitro. Subcellular co-expression of ZIP3 binding partners in the retina supported the proposed protein interactions. Our results indicate the formation of a ternary postsynaptic complex containing PKC-zeta ZIP3, and GABA(C) receptors.